Satellite images of city lights used to track measles

MEASLES hits migrating people when they crowd together. The good news is that it is possible to tell exactly where that is - and where to aim vaccination - by looking from space for the glow of the migrants' lights.

In 2008, after a vaccination programme had led to a global decline in deaths from the disease, measles bounced back. A resurgence among poor west African migrants, who tend to congregate during winter dry seasons, is a particular concern. However, because the virus also survives better in drier weather, it was hard to tell whether the crowding was the real problem.

Nita Bharti at Princeton University and colleagues have now teased apart the variables of crowding and weather by scrutinising satellite images for the glow of people's night-time fires and electric lights (Science, DOI: 10.1126/science.1210554).

In poor regions, night-time glow reveals population density, so Bharti's team used satellite images to map dry-season illumination increases in cities in Niger after dark. In three cities, the brighter periods coincided with recorded measles outbreaks. In a fourth, with similar weather but little seasonal migration, there was no increase in either glow or measles.

Claude Muller of the Institute of Immunology in Luxembourg says the insight into what makes measles seasonal is important. "If we understand that better, we can target vaccination better, to make sure less virus gets through the off season," which could help to eradicate the disease, he says.

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